As an error correcting method in data transmission, for example, there is a method called a type II hybrid ARQ method (Shuichi Sasaoka: Mobile Communications, page 240, Ohmsha,Ltd). In this error correcting method, only information bits are first transmitted, and then when it is necessary to perform retransmission with respect to the transmission (namely, when a receiving side cannot demodulate the information bits correctly), only parity bits (redundant bits) of FEC (Forward Error Correction) for error correction are retransmitted. The receiving side performs error correction using information bits previously received and the parity bits that have just received.
In the conventional data transmitting method, however, the transmission of information bits and the retransmission of parity bits are performed with the same power, causing a problem in which interference, which is equivalent to or more than interference caused at the time of transmitting information bits, will be given to other users at the time of retransmitting the parity bits. In particular, since the number of parity bits is generally smaller than that of information bits, transmission power per parity bit becomes higher than transmission power per information bit when transmission of information bits and that of parity bits are performed with the same power. This results in that the parity bits are retransmitted with transmission power more than necessary, thereby causing the aforementioned problem.
As a more specific explanation, for example, if information bits are transmitted with power P where the number of information bits is 1000 and that of parity bits is 10, power per bit becomes P/1000. Also, if the retransmission of parity bits is performed with the same power P, power per bit becomes P/10, resulting in an increase of transmission power as compared with information bits. In this way, in a case where transmission is performed with the same power P, power per bit of parity bits becomes higher than that of information bits, increasing in a possibility that interference, which is equivalent to or more than interference caused at the time of transmitting information bits, will be given to other users at the time of retransmitting the parity bits. Additionally, this problem occurs not only in the type II hybrid ARQ method but also in all methods in which the same power is used at the transmission data transmitting and retransmitting time.